Revert "Create new type definition for 'is-url' (#4013)"

This reverts commit 014b58cb40.

src/@types/is-url.d.ts

@@ -1,7 +0,0 @@
-declare module 'is-url' {
-	function isUrl(string: string): boolean;
-
-	namespace isUrl {} // Hack
-
-	export = isUrl;
-}

src/config/load.ts

@@ -6,7 +6,7 @@ import * as fs from 'fs';
 import { URL } from 'url';
 import * as yaml from 'js-yaml';
 import { Source, Mixin } from './types';
-import * as isUrl from 'is-url';
+import isUrl = require('is-url');
 import * as pkg from '../../package.json';
 
 /**